The Gate is a mysterious portal which appeared in the Ginza district of Tokyo in 20xx, which connected Japan to Alnus Hill, Special Region. As soon as it appeared, the Imperial Army invaded through the Gate and began to indiscriminately kill or capture every civilians they encountered, and overwhelming attempts by police forces to halt their advance failed.
It was at that time that Yōji Itami took command of the Tokyo police in the area and led an evacuation of civilians behind the walls of the Japanese Imperial Palace. From the palace, the police held off the attackers until the Japan Self-Defense Forces arrived and decisively defeated them. The incident would become known as the Battle of Ginza.
After the incident, the Gate was sealed off with a dome-shaped structure and JSDF forces were deployed to the other side of the Gate, marking the start of JSDF counterattack. Following the Japanese victory at the First and Second Battle of Alnus Hill, the JSDF constructed a base centered around the Gate, which was later expanded to include a refugee camp as well, becoming the community of Alnus. It is later revealed that Hardy, the Goddess of the Underworld of the Special Region is responsible for opening the Gate in her whim in order to destroy the old order of the Empire by making them face an enemy far superior to them; the JSDF.
Due to the fact that Gate only appears in Japan, it gives the country both huge advantages and disadvantages. The advantage of monopolizing the Gate enables Japan to have massive economic growth and influence in the global scale from accessing to massive untapped resources as well as a sole trader with locals in the Special Region. However, it is also putted Japan in the disadvantage position since the country now are being intimidated and pressured by a lot of superpowers like US, Russia, China or EU since these countries want a share of exploiting resources from the Special Region and trading with locals. The movement of the JSDF in mopping up the Imperial Troops during the early stage of the Japanese-Empire war and during the Imperial Civil War causing countries like Korea or Russia to accuse them as violating the treaty in WW2 about not creating an official army and becoming militarized. Despite the disadvantage, according to Akira Yaginada, Japan still has enough bargaining power against all countries in the Earth from their monopoly of the Gate.
After some times, the Gate randomly opens to the dimension of the Arachnid creating the devastating war for both the Empire and the JSDF until Lelei closes the gate with her magic to prevent more calamity. After Itami and some of the JSDF's personnel was trapped beyond the gate due to its closure, Lelei attempts to create a Gate with her magic to connect back to Japan. However, it is revealed that some superpower like US, Russia and China are secretly researching in constructing a dimensional gate through quantum physics to reach the Special Region without having to rely on Japan's Gate. So far, they have little success since the technology necessary for the dimensional gate is far too advanced.
It is revealed that if the Gate opens for too long it cause the strain in the flow of space and time stream causing catastrophic event in two worlds. The prime example is earthquake in Special Region.
During the event of Weight Anchor, Japan once again monopolizes the new Gate to exploit resources from the Avion Sea.
The Gate is a portal that exists between many worlds and the Special Region. It periodically opens and closes, bringing new races and cultures to across. The Gate is thought to be supernatural in nature (related to the gods) and responsible for the incredible, almost implausible diversity of races existing within the Special Region. The Gate always opens on Alnus Hill, explaining why all the races consider it a Holy Land and the origin of their species within their respective mythologies.
The Gate can happen randomly; being opened by a God, in this case Hardy or using magic in the case of Lelei. Furthermore, the Gate can be closed using magic or sealed through confined wall in the case when Gate threatens the time space continuum.